Due to the demand for four-wheel drive and all-wheel drive motor vehicles, a number of power transfer systems are currently incorporated into vehicular drivetrain applications for controlling the transfer of drive torque from the powertrain to the front and rear drivelines. A modern trend in such four-wheel drive vehicles is to provide a power transfer assembly having a transfer clutch that can be adaptively controlled to direct torque to the non-slipping wheels, without any input or action on the part of the vehicle operator, when traction is lost at the other wheels, thereby establishing an automatic or “on-demand” four-wheel drive mode. Typically, the transfer clutch includes a multi-plate friction clutch and a power-operated clutch actuator that is operable for generating and applying a clutch engagement force to the friction clutch. The power-operated clutch actuator usually includes a clutch operator unit and an electrically-powered device (i.e., electric motor, electromagnetic solenoid, etc.) that is actuated in response to control signals supplied by an electronic control system. The output of the electrically-powered device typically controls movement of the clutch operator unit (i.e., ball ramp unit, cam unit, etc.) for varying the magnitude of the clutch engagement force applied by the clutch operator unit to the friction clutch. Adaptive control of the electric control signal is typically based on changes in current operating and/or tractive conditions of the vehicle, as detected by a group of vehicle sensor associated with the control system. The control system for on-demand power transfer systems typically includes a dedicated electronic controller unit (ECU) programmed with adaptive clutch control logic for controlling the transfer of torque across the friction clutch during all types of driving and road conditions for providing enhanced vehicular traction and stability.
The control systems associated with most on-demand power transfer systems utilize the value of the speed differential between the front and rear drivelines as the primary characteristic for adaptively controlling the output signal supplied to the power-operated clutch actuator. However, the clutch control logic associated with such conventional control systems also typically utilizes one or more additional vehicular characteristics such as, for example, vehicle speed, steering angle, yaw rate, transmission gear ratio, brake status, throttle position and engine load when generating the output signal in an attempt to provide optimum traction/stability during most vehicle operating conditions.
While a wide variety of such power transfer systems and related clutch control schemes are currently used in four-wheel drive and all-wheel drive vehicles, a need exists to advance the technology and address recognized clutch control system limitations. Accordingly, the present invention addresses and improves upon conventional clutch control systems.